An optical cavity ring-down trace gas measurement system based on triangular ring cavity

Abstract

The invention discloses an optical cavity ring-down trace gas measurement system based on a triangular annular cavity, comprising an optical path module, an air path module, a data acquisition module and a control module. The optical path module is used to form a closed triangular optical path for detecting gas concentration, the air path module is used to pass a sample gas to be detected into the triangular optical path of the optical path module, the data acquisition module is used to collect the ring-down time of light in a cavity environment and an environment with gas absorption, and the control module is used to control the optical path module, the air path module and the data acquisition module. The invention increases the effective absorption path, improves the measurement lower limit, reduces the frequency fluctuation, improves the optical cavity coupling, reduces the baseline noise, and improves the absolute sensitivity. At the same time, because the light in the cavity is composed of traveling waves rather than standing waves, the uniformity of the light intensity in the cavity is improved, thereby reducing the error caused by uneven sample excitation.

Description

Technical Field

[0001] The present invention relates to the field of gas monitoring technology, and in particular to an optical cavity ring-down trace gas measurement system based on a triangular ring cavity. Background Art

[0002] The main spectral measurement methods for gases include tunable semiconductor laser absorption spectroscopy, photoacoustic spectroscopy, cavity enhanced absorption spectroscopy, Fourier infrared spectroscopy, non-dispersive infrared technology, differential absorption spectroscopy, off-axis integral cavity technology and cavity ring-down spectroscopy. Cavity ring-down spectroscopy is a high-precision measurement technology based on a ring-down cavity. It is a high-precision ring-down cavity composed of two or more highly reflective mirrors. The laser is coupled into the resonant cavity through mode matching to achieve coherent growth, which increases the absorption optical path. When the laser is quickly cut off, the detector detects the attenuation of the light intensity, and the cavity ring-down time is obtained by exponential fitting. The ring-down time is related to the reflectivity of the cavity mirror, the cavity length and the gas absorption. By measuring the difference in the ring-down time with and without gas, the concentration of the gas to be measured can be inverted, as shown in the following formula:

[0003]

[0004] In the above formula, C is the concentration of the gas to be measured, c is the speed of light, λ is the absorption cross section of the gas to be measured at a specific absorption peak, and T and T0 are the ring-down times in the presence of gas absorption and cavity, respectively.

[0005] In the prior art, most ring-down cavities are linear ring-down cavities, high-order modes are difficult to suppress, the signal-to-noise ratio of the ring-down signal is not high, the cavity structure is relatively long, and uneven sample excitation will cause an increase in errors. In summary, the present application now proposes an optical cavity ring-down trace gas measurement system based on a triangular ring cavity to solve the above-mentioned problems. Summary of the invention

[0006] In order to solve the above-mentioned deficiencies in the prior art, an object of the present invention is to provide an optical cavity ring-down trace gas measurement system based on a triangular ring cavity to solve the problems raised in the background technology.

[0007] The technical solution adopted by the present invention to solve its technical problems is: an optical cavity ring-down trace gas measurement system based on a triangular annular cavity, comprising an optical path module, a gas path module, a data acquisition module and a control module, wherein the optical path module is used to form a closed triangular optical path for detecting gas concentration, the gas path module is used to pass the sample gas to be detected into the triangular optical path of the optical path module, the data acquisition module is used to collect the ring-down time of light in a cavity environment and an environment with gas absorption, and the control module is used to control the optical path module, the gas path module and the data acquisition module.

[0008] Preferably, the optical path module includes a triangular annular cavity, a front optical path and a rear optical path. The cavity body of the triangular annular cavity is preferably made of invar with properties such as a small thermal expansion coefficient, a low thermal conductivity coefficient, and a low hardness. The diameters of the two ports of the cavity are different. The cavity body of the triangular annular cavity is provided with an air inlet for passing a sample gas to be tested and an air outlet corresponding to the air inlet. The planar positions of the air inlet and the air outlet are at least 2 cm apart from the first high-reflective mirror, the second high-reflective mirror and the third high-reflective mirror to avoid adverse effects of airflow on the reflection efficiency of the first high-reflective mirror, the second high-reflective mirror and the third high-reflective mirror. The triangular annular cavity comprises a first high-reflection mirror, a second high-reflection mirror and a third high-reflection mirror, wherein the reflectivity of the first high-reflection mirror, the second high-reflection mirror and the third high-reflection mirror are all greater than 99.99%, and the first high-reflection mirror, the second high-reflection mirror and the third high-reflection mirror are arranged in a triangular shape inside the cavity of the triangular annular cavity, and the first high-reflection mirror and the second high-reflection mirror are preferably plane mirrors, and are both bonded to one side end surface of the cavity of the triangular annular cavity, and the third high-reflection mirror is preferably a spherical mirror, and the third high-reflection mirror is bonded to the other side end surface of the cavity of the triangular annular cavity, and the angle between the first high-reflection mirror and the second high-reflection mirror is preferably designed to be 88.768° included angle, and the third high reflector is preferably arranged at the perpendicular midline of the line connecting the center points of the first high reflector and the second high reflector. The glue for bonding the first high reflector, the second high reflector and the third high reflector is preferably epoxy resin glue to ensure the airtightness of the cavity. The s-linear polarized light entering the triangular ring cavity is projected to the surface of the second high reflector through the first high reflector, and then reflected to the surface of the third high reflector. The third high reflector then projects the light beam to the surface of the first high reflector. The first high reflector reflects the light beam reflected back by the third high reflector to the surface of the second high reflector to form a closed triangular light beam. The pre-cavity optical path includes a laser, a first reflector, and a polarization beam splitter. The laser is used to output a light beam. The laser can use a DFB continuous wave laser, which has good monochromaticity, and its line width can generally be within 1MHz. The wavelength can be adjusted and has a very high side mode suppression ratio. The first reflector is used to change the light path direction of the light beam passing through the matching mirror. The polarization beam splitter is used to separate the s-linear polarized light and the p-linear polarized light. Since s-linear polarized light and p-linear polarized light may have phase mutation in the triangular ring cavity, resulting in that different polarized lights of the same beam cannot meet the resonance condition at the same time, thereby simultaneously exciting two intrinsic polarization state modes and destroying the intersection of the two intrinsic states, a polarization beam splitter is used to separate the s-linear polarized light and the p-linear polarized light, and the s-linear polarized light is injected into the triangular ring cavity; the post-cavity optical path includes a 1 / 4 glass plate and a second reflector, the 1 / 4 glass plate is used to transform the s-linear polarized light into circular polarized light or elliptically polarized light and to transform the circular polarized light or elliptically polarized light into P-linear polarized light, the second first reflector reflects light to the focusing mirror and focuses it on the end face of the photodetector, the triangular ring cavity outputs s-linear polarized light through the 1 / 4 glass plate, the 1 / 4 glass plate transforms the s-linear polarized light into circular polarized light or elliptically polarized light, the circular polarized light or elliptically polarized light is reflected back by the second reflector, and then passes through the 1 / 4 glass plate, the 1 / 4 glass plate transforms the circular polarized light or elliptically polarized light into p-linear polarized light, and does not interfere with the forward traveling wave. .

[0009] More preferably, the optical path module also includes an optical platform, and the pre-cavity optical path, post-cavity optical path and triangular annular optical cavity are all arranged on the optical platform. The optical platform is preferably made of tungsten copper, which has the characteristics of high hardness, low thermal expansion coefficient, good thermal conductivity, etc.

[0010] More preferably, the pre-cavity optical path also includes the first collimator, a matching mirror and a second collimator. The waist size of the laser output light beam can be adjusted by the first collimator or the first collimator and the second collimator together. The adjusted light beam passes through the matching mirror to match the waist of the triangular ring cavity. The lens parameters of the first collimator and the second collimator are determined according to the collimation principle of Gaussian beam.

[0011] More preferably, the pre-cavity optical path also includes a prism, which is used to adjust the spatial position of the light beam, to adjust the angle of the light beam input into the triangular ring cavity, and to adjust the angle of the light beam incident into the triangular ring cavity to ensure that the angle of the incident light beam meets the conditions for forming a closed optical path.

[0012] More preferably, the data acquisition module includes a photodetector and a focusing mirror, the photodetector is used to collect the ringing time of light in a cavity environment and a gas absorption environment, the light reflected by the second reflector is focused by the focusing mirror on the end face of the photodetector, the photodetector can be connected to an external acquisition card through a data cable, and the acquisition card is used to store and transmit the detected data.

[0013] More preferably, the air circuit module includes an air pump, two proportional valves, a primary filter and a secondary filter, the air inlet pipe of the air pump is opened and closed by one of the proportional valves, the air inlet pipe of the air pump passes through the primary filter and the secondary filter, and is connected to the air inlet of the triangular annular cavity, the air outlet pipe of the air pump is connected to the air outlet of the triangular annular cavity, the air outlet pipe of the air pump is opened and closed by another proportional valve to control the opening and closing of the passage, the air pump provides power for the sample gas to be tested to enter and exit the triangular annular cavity, the two proportional valves control the opening and closing of the air inlet pipe of the air pump, and the primary filter and the secondary filter filter the sample gas.

[0014] More preferably, the control module includes a laser driver, a cavity temperature controller and an electric shutoff, the laser is controlled by the laser driver, the cavity temperature controller is used to control the internal temperature of the triangular ring cavity, and the electric shutoff controls the shutdown of the laser.

[0015] From the above, it can be seen that the invention has the following beneficial effects: the invention uses a triangular ring cavity to allow the laser to circulate and reflect in the ring-down cavity, thereby increasing the effective absorption path and improving the lower limit of measurement; because the incident light and the return light of the triangular cavity are in different directions, there is no feedback to the light source, thus reducing frequency fluctuations, improving optical cavity coupling, reducing baseline noise, and improving absolute sensitivity. At the same time, because the light in the cavity is composed of traveling waves rather than standing waves, the uniformity of the light intensity in the cavity is improved, thereby reducing the error caused by uneven sample excitation. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a module schematic diagram of a triangular ring cavity-based optical cavity ring-down trace gas analysis system provided by an embodiment of the present invention;

[0017] Figure 2 This is a schematic diagram of the optical path structure of an optical cavity ring-down trace gas analysis system based on a triangular ring cavity provided by an embodiment of the present invention;

[0018] Figure 3This is a schematic diagram of beam matching of a cavity ring-down trace gas analysis system based on a triangular ring cavity provided by an embodiment of the present invention;

[0019] Figure 4 It is a structural schematic diagram of a triangular ring cavity of an optical cavity ring-down trace gas analysis system based on a triangular ring cavity provided in an embodiment of the present invention.

[0020] In the accompanying drawings: 1. pre-cavity optical path; 2. triangular ring cavity; 3. post-cavity optical path; 4. optical platform; 5. data acquisition module; 6. gas path module; 7. laser driver; 8. electric shut-off device; 9. cavity temperature controller; 11. laser; 12. first collimator; 13. second collimator; 14. matching mirror; 15. first reflector; 16. polarization beam splitter; 17. prism; 201. first high-reflection mirror; 202. second high-reflection mirror; 203. third high-reflection mirror; 204. air outlet; 205. air inlet; 31. 1 / 4 glass slide; 32. second reflector; 33. focusing mirror; 51. photodetector. DETAILED DESCRIPTION

[0021] The present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant invention, rather than to limit the invention. It is also necessary to explain that, for ease of description, only the parts related to the invention are shown in the accompanying drawings. Example

[0022] See also Figure 1-Figure 4 A cavity ring-down trace gas measurement system based on a triangular annular cavity comprises an optical path module, a gas path module 6, a data acquisition module 5 and a control module. The optical path module is used to form a closed triangular optical path for detecting gas concentration. The optical path module comprises a triangular annular cavity 2, a pre-cavity optical path 1 and a post-cavity optical path 3. The pre-cavity optical path 1 comprises a laser 11, a first collimator 12, a second collimator 13, a matching mirror 14, a first reflector 15, a polarization beam splitter 16 and a prism 17. The laser 11 emits a Gaussian beam, which changes the beam waist size after passing through the first collimator 12 and the second collimator 13, and then passes through the matching mirror 14 to match the beam waist with the beam waist of the triangular annular cavity 2. After the beam is reflected by the first reflector 15, it reaches the polarization beam splitter 16, which separates the s-linear polarized light and the p-linear polarized light and injects the s-linear polarized light into the triangular annular cavity.

[0023] The triangular annular cavity 2 comprises a first high reflective mirror 201, a second high reflective mirror 202 and a third high reflective mirror 203, which are arranged in a triangular shape inside the cavity of the triangular annular cavity 2. The first high reflective mirror 201 and the second high reflective mirror 202 are preferably plane mirrors, and are both bonded to one side end face of the cavity of the triangular annular cavity 2. The third high reflective mirror 203 is preferably a spherical mirror, and is bonded to the other side end face of the cavity of the triangular annular cavity 2. The s-linear polarized light entering the triangular annular cavity 2 passes through the first high reflective mirror 201 and is projected to the surface of the second high reflective mirror 202, and then is reflected to the surface of the third high reflective mirror 203. The third high reflective mirror 203 then projects the light beam to the surface of the first high reflective mirror 201. The first high reflective mirror 201 reflects the light beam reflected back by the third high reflective mirror 203 to the surface of the second high reflective mirror 202, thereby forming a closed triangular light beam.

[0024] The post-cavity optical path 3 includes a 1 / 4 glass slide 31 and a second reflector 32. After the s-linear polarized light passing through the triangular ring cavity 2 passes through the surface of the 1 / 4 glass slide 31, the s-linear polarized light is converted into circularly polarized light and reaches the second reflector 32. The second reflector 32 reflects the s-linear polarized light back into the triangular ring cavity 2. On the way, after passing through the other side surface of the 1 / 4 glass slide 31, the 1 / 4 glass slide 31 converts the circularly polarized light into p-linear polarized light and injects it into the triangular ring cavity 2 again. Similarly, the p-linear polarized light forms a reverse circular closed optical path in the triangular ring cavity 2 in the opposite direction to the s-linear polarized light. At the same time, when the p-linear polarized light passes through the polarization beam splitter 16, it will be refracted by the polarization beam splitter 16 and will not interfere with the light source.

[0025] The cavity of the triangular annular cavity 2 is provided with an air inlet 205 for introducing the sample gas to be tested and an air outlet 204 corresponding to the air inlet 205. The air path module 6 is used to introduce the sample gas to be tested into the triangular optical path of the optical path module. The air path module 6 includes an air pump, an air pump, two proportional valves, a primary filter and a secondary filter. The air inlet pipeline of the air pump is opened and closed by one of the proportional valves. The air inlet pipeline of the air pump passes through the primary filter and the secondary filter, and is connected to the air inlet 205 of the triangular annular cavity 2. The outlet of the air pump The air pipeline is connected to the air outlet 204 of the triangular annular cavity 2, and the air outlet pipeline of the air pump is controlled to open and close by another proportional valve. The air pump provides power for the sample gas to be detected to enter and exit the triangular annular cavity 2. The primary filter and the secondary filter filter the sample gas. The air pump provides power for the sample gas to be detected to be discharged from the triangular annular cavity 2. The air pump injects the sample gas to be detected into the triangular annular cavity 2 through the air inlet 205 of the triangular annular cavity 2, and the air pump can discharge the gas inside the triangular annular cavity 2 from the air outlet 204 of the triangular annular cavity 2.

[0026] The data acquisition module 5 is used to collect the ring-down time of light in the cavity environment and the gas absorption environment. The data acquisition module 5 includes a photodetector 51 and a focusing mirror 33. The photodetector 51 collects the ring-down time of light in the cavity environment and the gas absorption environment. The light reflected by the second reflector 32 is focused on the end face of the photodetector 51 by the focusing mirror 33. The photodetector 51 can be connected to an external acquisition card through a data line. The acquisition card is used to store and transmit the detected data. After recording the data, the concentration of the gas to be measured is calculated by the following formula:

[0027] ;

[0028] Where C is the concentration of the gas to be measured, c is the speed of light, λ is the absorption cross section of the gas to be measured at a specific absorption peak, and T and T0 are the ring-down times in the presence of gas absorption and cavity, respectively. Example

[0029] The first high reflective mirror 201 and the second high reflective mirror 202 are preferably plane mirrors, and are both bonded to one side end face of the cavity of the triangular annular cavity 2. The third high reflective mirror 203 is preferably a spherical mirror, and the third high reflective mirror 203 is bonded to the other side end face of the cavity of the triangular annular cavity 2. The angle between the first high reflective mirror 201 and the second high reflective mirror 202 is preferably designed to be 88.768°, and the third high reflective mirror 203 is preferably arranged at the perpendicular bisector of the line connecting the center points of the first high reflective mirror 201 and the second high reflective mirror 202.

[0030] The waist radius of the Gaussian beam output by the laser 11 is determined by the output optical fiber diameter, which is very small. The waist radius and position of the Gaussian beam in the triangular ring cavity 2 are determined by the long and short sides of the triangular closed optical path and the curvature radius R of the spherical high-reflection mirror. The waist position of the Gaussian beam in the cavity is located in the middle position between the input plane mirror 701 and the output plane mirror 702.

[0031] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0032] The above description is only a preferred embodiment of the present application and an explanation of the technical principles used. Those skilled in the art should understand that the scope of the invention involved in the present application is not limited to the technical solution formed by a specific combination of the above technical features, but should also cover other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the inventive concept. For example, the above features are replaced with (but not limited to) technical features with similar functions disclosed in the present application.

[0033] Except for the technical features described in the specification, the remaining technical features are known technologies to those skilled in the art. In order to highlight the innovative features of the present invention, the remaining technical features will not be described here in detail.

Claims

1. An optical cavity ring-down trace gas measurement system based on a triangular ring cavity, characterized in that: include: An optical path module, wherein the optical path module is used to form a closed triangular optical path for detecting gas concentration; A gas path module (6), the gas path module (6) being used to introduce a sample gas to be detected into the interior of the triangular optical path of the optical path module; A data acquisition module (5), the data acquisition module (5) being used to collect the ring-down time of light in a cavity environment and an environment with gas absorption; as well as A control module, the control module being used to control the optical path module, the gas path module (6) and the data acquisition module (5); The optical path module comprises a pre-cavity optical path (1), a triangular ring cavity (2) and a post-cavity optical path (3): The triangular annular cavity (2) is provided with an air inlet (205) for introducing a sample gas to be tested and an air outlet (204) corresponding to the air inlet (205); the triangular annular cavity (2) comprises a first high-reflection mirror (201), a second high-reflection mirror (202) and a third high-reflection mirror (203); the first high-reflection mirror (201), the second high-reflection mirror (202) and the third high-reflection mirror (203) are arranged in a triangular shape inside the cavity of the triangular annular cavity (2); the first high-reflection mirror (201) and the second high-reflection mirror (202) are both plane high-reflection mirrors, the third high-reflection mirror (203) is a spherical high-reflection mirror, and the angle between the first high-reflection mirror (201) and the second high-reflection mirror (202) is 85°-90°; The pre-cavity optical path (1) comprises a laser (11), a first reflector (15), a polarization beam splitter (16) and a matching mirror (14); the laser (11) is used to output a light beam; the first reflector (15) is used to change the light path direction of the light beam passing through the matching mirror (14); the polarization beam splitter (16) is used to separate s-linear polarized light and p-linear polarized light; The pre-cavity optical path (1) further comprises a first collimator (12), wherein the first collimator (12) is used to change the beam waist size of the output beam of the laser (11); the matching mirror (14) is used to adjust the beam waist of the first collimator (12); The post-cavity optical path (3) comprises a quarter glass plate (31) and a second reflector (32); the quarter glass plate (31) is used to transform s-linear polarized light into circularly polarized light or elliptically polarized light, and to transform circularly polarized light or elliptically polarized light into P-linear polarized light; and the second reflector (32) is used to reflect the light beam output by the triangular ring cavity (2) back into the interior of the triangular ring cavity (2).

2. According to claim 1, the optical cavity ring-down trace gas measurement system based on the triangular ring cavity is characterized in that: The optical path module also comprises an optical platform (4), and the pre-cavity optical path (1), the post-cavity optical path (3) and the triangular annular optical cavity are all arranged on the optical platform (4).

3. The optical cavity ring-down trace gas measurement system based on a triangular ring cavity according to claim 2 is characterized in that: The pre-cavity optical path (1) further comprises a second collimator (13), and the second collimator (13) cooperates with the first collimator (12) to change the beam waist size of the output light beam of the laser (11).

4. The optical cavity ring-down trace gas measurement system based on a triangular ring cavity according to claim 3 is characterized in that: The pre-cavity optical path (1) further comprises a prism (17), wherein the prism (17) is used to adjust the spatial position of the light beam, and is used to adjust the angle of the light beam input into the triangular ring cavity (2).

5. The optical cavity ring-down trace gas measurement system based on a triangular ring cavity according to claim 4 is characterized in that: The data acquisition module (5) comprises: A photodetector (51), the photodetector (51) being used to collect the ring-down time of light in a cavity environment and an environment with gas absorption; and A focusing mirror (33), wherein the light reflected by the second reflecting mirror (32) is focused by the focusing mirror (33) onto the end surface of the photodetector (51).

6. The optical cavity ring-down trace gas measurement system based on a triangular ring cavity according to claim 5 is characterized in that: The air circuit module (6) comprises an air pump, two proportional valves, a primary filter and a secondary filter; the air inlet pipeline of the air pump is controlled by one of the proportional valves to open and close the passage; the air inlet pipeline of the air pump passes through the primary filter and the secondary filter and is connected to the air inlet (205) of the triangular annular cavity (2); the air outlet pipeline of the air pump is connected to the air outlet (204) of the triangular annular cavity (2) and the opening and closing of the passage is controlled by another proportional valve.

7. The optical cavity ring-down trace gas measurement system based on a triangular ring cavity according to claim 6 is characterized in that: The control module comprises: A laser driver (7), wherein the laser (11) is controlled to operate by the laser driver (7); A cavity temperature controller (9), the cavity temperature controller (9) being used to control the internal temperature of the triangular annular cavity (2); and An electric switch (8) controls the switching off of the laser (11).

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